https://www.musictheory.net/

 This extensive site starts with the very basics and continues through advanced music theory. There are tutorials and games for beginning levels.

https://www.youtube.com/user/the1564studios/playlists

This site has short music tutorials on various music topics.

http://jamzone.littlekidsrock.org/instrument/vocals/

There are TONS of options to explore for all ages levels.